Effect on the mechanical properties of rabbit lungs of varying tidal volume, end-expiratory pressure, and composition of breathing gas are being investigated. The dependence of the shape of the flow-volume curves on thoracotomy, denervation, and pharmacological agents affecting smooth muscle are also to be assessed. The shift in the pressure-volume curve seen in acute asthma will be examined in animals. Comparative studies of elastic, hysteretic, and dynamic airway characteristics between various species of rodents, carnivores and primates will be undertaken.